The steel used for preparation of vehicle outer panels such as doors, hoods, and fenders is required on one hand to be soft and exhibits an excellent formability before formation of the outer panel and is also required on the other hand to exhibit a high mechanical strength after the panel forming and painting so as to improve the dent resistance.
In recent years, a bake-hardenable (BH) cold rolled steel sheet has been developed. as a steel sheet meeting these requirements and is widely used nowadays. Particularly, vigorous studies are being made on a method of manufacturing a BH steel sheet having an excellent deep-drawability by annealing a cold rolled steel sheet prepared by adding carbonitriding elements such as Nb and Ti at an atomic ratio of 1 or less in terms of the carbon equivalent to an extra low carbon steel having the carbon content lowered to about 50 ppm.
For example, Japanese Patent Publication (Kokoku) No. 60-46166 discloses a technique of annealing an extra low carbon steel having Nb or Ti added thereto at a high temperature, i.e., about 900.degree. C. The technique disclosed in this prior art is desirable in that, since the steel is annealed at a high temperature, it is possible to improve the bake-hardenability (BH), i.e., the degree of hardening of the steel that can be achieved when a strained steel is baked, and to improve an r-value providing an index of deep-drawability. However, this technique gives rise to a difficulty that the ferrite grains may be coarsened so as to deteriorate the surface state of the steel sheet. Also, since the steel sheet itself is softened, the mechanical strength of the steel sheet after the forming and painting is not satisfactory, even if high BH property is imparted to the steel sheet.
Japanese Patent Disclosure (Kokai) No. 61-276928 discloses a technique of manufacturing a BH steel sheet by annealing an extra low carbon steel having Nb added thereto at a temperature range of about 700 to 850.degree. C. The annealing temperature disclosed in this prior art is relatively low, compared with that disclosed in JP '166 noted above. The low annealing temperature is desirable in respect of the surface and the yield strength of the steel sheet, but is not satisfactory for improving the r-value and BH property.
Further, although each of the prior arts exemplified above is intended to improve the BH property of a steel sheet, the improvement in the BH property promotes the natural aging. It follows that a yield point elongation is likely to be generated during the storage at room temperature, with the result that a stretcher strain may be generated in the panel forming. In order to prevent these difficulties, an improvement in the BH property is limited.